1. Field of the Invention
This invention concerns a detection device for contact fusion which detects the contact fusion of main contacts in an electromagnetic contactor by detecting a voltage of main contacts on the load side and a voltage applied to an operation coil in an electromagnetic contactor for use which an induction motor conducting inching or plugging and which thereby tripping a breaker.
2. Description of the Prior Art
Heretofore, there has been known, as shown in FIG. 3, a detection device 10 for contact fusion in an electromagnetic contactor of the kind as described above having three current transformers 6, 7 and 8 and a logic circuit 9 including relay contacts 11 for detecting the contact fusion. In this device, a breaker 1 equipped with an electronic tripping device has three phase power supply terminals 41, 42, 43 on the primary side thereof which are connected respectively to R-phase, S-phase and T-phase of three phases. The secondary side of the breaker 1 is connected at the U-phase and at the V-phase by way of the primary side of the current transformers 7 and 8 respectively and at the W-phase directly, to the terminals of main contacts 3 of an electromagnetic contactor 12 connected at its other ends to an electric motor 4. Further, the U-phase on the secondary side of the breaker 1 is connected by way of the primary side of the current transformer 6, a pushbutton switch 5 and an operation coil 2 of an electromagnetic contactor 12 to the V-phase. Further, the logic circuit 9 is connected to the secondary side of the current transformers 6, 7, 8 and the relay contacts 11 for detecting the contact fusion that are opened and closed depending on the judgement of the logic circuit 9 are connected in series with the tripping coil 1a of the breaker 1 and disposed between the U- and V-phases on the secondary side of the breaker 1. If fusion has occurred in the main contacts 3 of the electromagnetic contactor 12, current flows through the current transformers 7 and 8 even if the pushbutton switch 5 is turned OFF, which is compared with the current transformer 6 in which no current flows when the pushbutton switch 5 is turned OFF by the logic circuit 9 to turn the relay contact 11 for the contact fusion as the result of the judgement thereby exciting the tripping coil 1a of the breaker 1 to trip the breaker 1, whereby the electrical motor 4 is stopped and the fusion in the main contacts 3 is detected. In the detection device for contact fusion in the electromagnetic contactor as described above, current transformers 6, 7, 8 are necessary for detecting the current flowing to the electric motor 4 and the current flowing to the operation coil 2 of the electromagnetic contactor 12, and the size of the current transformers 7 and 8 is enlarged along with the increase in the capacity of the electric motor 4 to make the detection device for contact fusion larger in the case of a large current electric motor 4. While on the other hand, the logic circuit 9 does not operate for a motor 4 with only a small current, thereby making it difficult to detect the fusion of the contacts.